Determining a plane of an anatomical body part

ABSTRACT

A data processing method for determining the position of a main plane of an anatomical body part, comprising the steps of: ·providing absolute auxiliary point data which describe the position of at least one actual auxiliary point of the body part relative to a marker device attached to the body part, the at least one actual auxiliary point being outside the main plane; ·providing relative point data which constrain the possible positions of the main plane relative to the at least one actual auxiliary point; ·providing absolute main point data which describe the position of one or two actual main points of the body part relative to the marker device attached to the body part, said one or two actual main points lying in the main plane and/or calculating the position of at least one virtual main point relative to the marker device, said at least one virtual main point being in the main plane and being calculated based on the absolute auxiliary point data and the relative point data; ·calculating a position of the main plane relative to the marker device, wherein the calculation uses the relative point data and auxiliary point data as well as the provided absolute main point data and/or the calculated position of the at least one virtual main point.

The present invention relates to determining a plane of an anatomicalbody part (for example a pelvis or head) which shall be referred to inthe following as a main plane. The “anatomical body part” shall also bereferred simply as the “body part” for short, and the “anatomicalpelvis” simply as the “pelvis”. A main plane is in particular a plane ofan anatomical body part which main plane is used in surgery (for examplein brain or hip surgery) as a reference for defining the positions ofother body parts (for example the acetabulum), in particular of otheranatomical structures. A main plane is in particular a plane which canbe defined by anatomical landmarks of the anatomical body part (forexample the pelvis). Planes which can be defined by these landmarksand/or which are used for defining the position of the femur oracetabulum are in particular the mid-sagittal plane (MSP) or theanterior pelvic plane (APP).

The main plane and the auxiliary plane are in particular not paralleland are in particular constituted to define the position of a referencesystem, in particular coordinate system. Generally speaking, at leasttwo planes of the anatomical body part (for example the pelvis) aredefined in order to determine a reference system (coordinate system)with respect to which the above-mentioned position of another anatomicalbody part (e.g. femur or the acetabulum) can be defined. One of theseplanes is the aforementioned main plane (for example the MSP), the otherplane can be a plane referred to here as the auxiliary plane (forexample the spinae joint center plane or SJCP to be defined later) orcan be a plane referred to here as the standard plane (for example theAPP). Like the main plane, the auxiliary plane, and standard plane arepreferably defined by landmarks of the anatomical body part (for examplethe pelvis).

It is advantageous to determine the position of the main plane reliablyand with a sufficient degree of accuracy to allow the position of theother body parts, in particular anatomical structures (for example theacetabulum) to be determined with a sufficient degree of accuracy.

In accordance with a common prior-art method, the pelvis is registeredin a supine position in particular for hip surgeries and then turnedover to a lateral position. In the supine position, landmarks aredetected by means of a pointer in order to determine the location oflandmarks on the MSP and APP relative to a marker device fixed to thepelvis.

Reference is also made to U.S. 2008/132783 A1. In accordance with thispatent application, points are determined in three cardinal planes.Reference is also made to U.S. 2003/0153829, U.S. 2002/0077540 and WO2005/084541.

The object of the invention is to allow a position, in particular anorientation of a main plane of an anatomical body part (for example apelvis) to be determined on the basis of the position of (main andauxiliary) points which in particular represent landmarks. Preferably,all represented landmarks can be detected (scanned) when the patient isin one position, in particular the lateral position.

The above object is solved by the subject-matter of the independentclaims. The dependent claims are directed to advantageous embodiments.

As far as herein the “position” of a plane is mentioned, the termposition can mean absolute location of the plane or just orientation ofthe plane. In particular determination of the latter is often sufficientin hip surgery since the orientation of the plane relative to theorientation of the acetabulum is of main interest for the surgeon.

One advantage of the invention is that it is not necessary to repositionthe anatomical body part (for example the pelvis) and thus the patienton the table. Furthermore, the present invention relies in particular onlandmarks which are easily accessible for a surgeon. In particular,points which are symmetrical to the auxiliary points with respect to themain plane and remote from the main plane (for example the ASIS point)can be detected but do not have to be detected. These symmetrical and/orremote points can be difficult to detect.

Advantageously, determining the main plane does not rely on fluoroscopicimages produced during surgery. Thus, the exposure of the patient andoperating theatre staff to radiation is reduced.

Advantageously, the present invention can rely on position data forlandmarks which can be detected to a high degree of accuracy and whichare in particular not covered by soft tissue or fat. These position dataare in particular detected by means of a pointer (see below).

A method in accordance with the invention is in particular a dataprocessing method or comprises the data processing method. The dataprocessing method is preferably performed using technical means, inparticular a computer. The computer in particular comprises a processorand a memory in order to process the data, in particular electronically.The calculating steps described are in particular performed by acomputer. Steps of determining or calculating are in particular steps ofdetermining data within the framework of the technical data processingmethod, in particular within the framework of a program. A computer isin particular any kind of data processing device. A computer can be adevice which is generally thought of as such, for example desktop PCs ornotebooks or netbooks, etc., but a computer can be also any programmableapparatus, such as a mobile phone or an embedded processor. Inparticular, a computer can comprise a system (network) of“sub-computers”, wherein each sub-computer represents a computer in itsown right. A computer in particular comprises interfaces in order toreceive data and/or to perform an analog-to-digital conversion.

Merely as a non-limiting example of an anatomical body part, the term“anatomical pelvis” or “pelvis” is used in the following instead of“anatomical body part”.

The data processing method of the present invention is a method fordetermining the position, in particular orientation of the main plane(e.g. MSP) of the anatomical pelvis. The pelvis is in particular thepelvis of a subject (patient) who is in particular lying in a lateralposition. The position, in particular orientation of the main plane isin particular determined relative to a marker device which is attachedto the pelvis.

A marker device can for instance be a reference star or a pointer or oneor more (individual) markers in a predetermined spatial relationship. Amarker device comprises one, two, three or more markers in apredetermined spatial relationship. This predetermined spatialrelationship is in particular known to a navigation system, for examplestored in a computer of the navigation system.

The function of a marker is to be detected by a marker detection device(for example, a camera or an ultrasound receiver), such that its spatialposition (i.e. its spatial location and/or alignment) can beascertained. The detection device is in particular part of a navigationsystem. The markers can be active markers. An active marker emits forexample electromagnetic radiation and/or waves, wherein said radiationcan be in the infrared, visible and/or ultraviolet spectral range. Themarker can also however be passive, i.e. can for example reflectelectromagnetic radiation from the infrared, visible and/or ultravioletspectral range. To this end, the marker can be provided with a surfacewhich has corresponding reflective properties. It is also possible for amarker to reflect and/or emit electromagnetic radiation and/or waves inthe radio frequency range or at ultrasound wavelengths. A markerpreferably has a spherical and/or spheroid shape and may therefore bereferred to as a marker sphere; markers can also, however, exhibit acornered—for example, cubic—shape.

A “reference star” refers to a device which a number of markers,advantageously three markers, are attached to, wherein the markers are(in particular detachably) attached to the reference star such that theyare stationary, thus providing a known (and advantageously fixed)position of the markers relative to each other. The position of themarkers relative to each other can be individually different for eachreference star used within the framework of a surgical navigationmethod, in order to enable the corresponding reference star to beidentified by a surgical navigation system on the basis of the positionof the markers relative to each other. It is thus also then possible forthe objects (for example, instruments and/or parts of a body) to whichthe reference star is attached to be identified and/or differentiatedfrom each other. In a surgical navigation method, the reference starserves to attach a plurality of markers to an object (for example, abone or a medical instrument) in order to be able to detect the positionof the object (i.e. its spatial location and/or alignment). Such areference star in particular comprises a way of being attached to theobject (for example, a clamp and/or a thread) and/or a holding elementwhich ensures a distance between the markers and the object (inparticular in order to assist the visibility of the markers to a markerdetection device) and/or marker holders which are mechanically connectedto the holding element and which the markers can be attached to.

The position of the main plane is in particular described in a referencesystem (coordinate system) of a navigation system, in particular asurgical navigation system (also referred to as a computer-assistednavigation system or image-guided surgery system).

A navigation system, in particular a surgical navigation system, isunderstood to mean a system which may comprise: at least one markerdevice; a transmitter which emits electromagnetic waves and/or radiationand/or ultrasound waves; a receiver which receives electromagnetic wavesand/or radiation and/or ultrasound waves; and an electronic dataprocessing device which is connected to the receiver and/or thetransmitter, wherein the data processing device (for example, acomputer) comprises in particular a processor (CPU), a working memory,advantageously an indicating device for issuing an indication signal(for example, a visual indicating device such as a monitor and/or anaudio indicating device such as a loudspeaker and/or tactile indicatingdevice such as a vibrator) and advantageously a permanent data memory,wherein the data processing device processes navigation data forwardedto it by the receiver and can advantageously output guidance informationto a user via the indicating device. The navigation data can be storedin the permanent data memory and for example compared with data whichhave been stored in said memory beforehand.

According to an embodiment of the invention, absolute main point dataare provided. These absolute main point data can describe the positionof one (actual) main point or the position of two (actual) main pointsof the pelvis relative to the marker device which is in particularattached to the pelvis or of more (actual) main points. In particular,the absolute main point data describe the position of only one mainpoint of the pelvis or the position of only two main points of thepelvis relative to the marker device. The main points are points whichlie in the main plane and the position of which is in particular definedby landmarks of the pelvis. Generally the main points can lie anywhereon the main plane. According to another embodiment, no absolute mainpoint data are provided but instead virtual main points are calculatedbased on the absolute auxiliary point data and the relative point data.The virtual main points lie in the main plane and can but have not tolie in the body part. These calculated virtual main points can be usedfor the further determination, in particular calculation in accordancewith the described inventive method in the same way as the actual mainpoints described by the absolute main pint data. According to a furtherembodiment both the provided actual main points and the calculatedvirtual main points are used for the further determination in accordancewith the invention.

A landmark is a defined position of an anatomical characteristic of ananatomical body part which is always identical or recurs with a highdegree of similarity in the same anatomical body part of multiplepatients. Typical landmarks are for example the anterior superior iliacspine (ASIS) points or the tips of the dorsal process of a vertebra. Thepoints (main points or auxiliary points) can represent such landmarks. Alandmark which lies on (in particular on the surface of) acharacteristic anatomical structure of the body part can also representsaid structure. The landmark can represent the anatomical structure oronly a point or part of it. For instance, a landmark can also lie on theanatomic structure which is in particular a prominent structure. Anexample of such an anatomic structure is the posterior aspect of theiliac crest. Other landmarks include a landmark defined by the rim ofthe acetabulum, for instance by the center of the rim. Another exampleis where a landmark represents the bottom or deepest point of anacetabulum, which is derived from a multitude of detection points. Thus,one landmark can in particular represent a multitude of detectionpoints. As mentioned above, a landmark can represent an anatomicalcharacteristic which is defined based on a characteristic structure ofthe body part. Additionally, a landmark can also represent an anatomicalcharacteristic defined by a relative movement of two body parts, such asthe rotational center of the femur when moved relative to theacetabulum.

A detection point is in particular a point on the surface of theanatomical structure which is detected, for example by a pointer.

Absolute auxiliary point data are also provided in accordance with theinvention. These absolute auxiliary point data describe the position ofat least one auxiliary point of the pelvis relative to the markerdevice. Unlike the main points, an auxiliary point lies outside the mainplane. If there is more than one auxiliary point, the auxiliary pointslie in particular in the so-called auxiliary plane (already mentionedabove), as will be explained in more detail below. The auxiliary pointsrepresent the position of the anatomical characteristics and are inparticular (directly or indirectly) defined by landmarks. Both the mainpoints and the auxiliary points are in particular, but not necessarily,points lying on the surface of the pelvis. As mentioned above, the mainpoints and auxiliary points can also be defined indirectly, as will beexplained in more detail below, for instance by the rim of theacetabulum or by a rotation center or by landmarks which are symmetricalrelative to a plane (for example the main plane or auxiliary plane),again to name but a few examples of indirect definitions of auxiliarypoints or main points.

According to one advantageous embodiment of a determination method inaccordance with the invention, the position of the main points and/orauxiliary points (which represent landmarks) can be detected by a stepof contacting the pelvis with the above-mentioned pointer which ishandled by a medical assistant and brought into contact with the pelvis.This optional detection step allows the absolute main point data andabsolute auxiliary point data to be provided and in particular does notform a part of the claimed data processing method but can form a part ofa general determination method for determining the position of the mainplane. The step of detecting in particular does not encompass orcomprise an invasive step representing a substantive physicalintervention on the body which requires professional medical expertiseto be carried out and/or which entails substantial health risk even whencarried out with required professional care and expertise. The step ofdetecting does in particular not comprise or encompass a surgical step.The step of detecting does in particular not comprise or encompass astep for treatment of a human or animal body by surgery or therapy. Thestep of providing the absolute main point data and absolute auxiliarypoint data includes in particular a step of receiving the absolute mainpoint data and absolute auxiliary point data by the data processingmethod. The received data have been in particular generated by thedetection step. The detection step is in particular not part of the dataprocessing method but is optionally part of the determination method inaccordance with the invention. As mentioned above, a multitude ofdetected points (detection points) on the surface of the pelvis canresult in the position of just one landmark and therefore just one mainpoint and/or auxiliary point being detected. A multitude of detectionpoints are for example necessary in order to determine the deepest point(fossa) in the acetabulum which is a landmark. Another example is when amultitude of detection points are necessary in order to define severalpositions of the femur relative to the acetabulum. This allows thecenter of rotation of the relative movement of the femur to bedetermined. This center of rotation is then a landmark.

The data processing method of the present invention is directed to dataprocessing and in particular does not include steps relating tocontacting an anatomical structure.

Relative point data are also provided in accordance with the invention.These relative point data constrain the possible position of the mainplane relative to the at least one auxiliary point, in particularrelative to only one of the one or more auxiliary points and/or relativeto only two of the auxiliary points. The relative point data can inparticular constrain the possible positions between a particularauxiliary point of the one or more auxiliary points and another pointreferred to as the virtual auxiliary point, which is not included in theabsolute auxiliary point data. This virtual auxiliary point is inparticular a point which is symmetrical to the particular auxiliarypoint of the one or more auxiliary points with regard to the main plane.Thus, the possible positions of the main plane relative to the at leastone auxiliary point are in particular constrained by constraining thepossible positions between the at least one auxiliary point and thevirtual auxiliary point and/or between the at least one auxiliary pointand a virtual main point (see below). An example of such a constraint isthat the auxiliary point and its corresponding (symmetrical) virtualpoint must have a certain distance. A further example of the relativepoint data is that these data describe an angle between two linesdefined by at least three points, at least one of these three points canbe a main point (virtual or actual main point). Thus also positionalrelationships, in which (virtual or actual) main points are involved,can represent constraints for the possible positions of the main planerelative to the at least one auxiliary point. The absolute main pointdata describe in particular the position of one or two actual mainpoints while they do not describe the position of the virtual mainpoints. The virtual main points are in particular determined, inparticular calculated by means of the data processing method. The actualmain points are in particular received by the data processing method asmentioned above. Correspondingly, the absolute auxiliary point datadescribe the position of at least one actual auxiliary point while theydo not in particular describe the position of virtual auxiliary points.The virtual auxiliary points are in particular determined, in particularcalculated by means of the data processing method.

The constraints described by the relative point data are in particularrepresented by one or two or more scalar values. These scalar values arein particular used for describing a positional relationship between themain plane and a zero-dimensional or one-dimensional geometrical object,such as a point or line, or between two such geometrical objects. Theconstraints given by the relative point data are in particularincomplete, i.e. do not allow the exact position of the main planerelative to the at least one auxiliary point to be calculated in areference system (coordinate system) based only on the position of theat least one auxiliary point and the relative point data. Additionalinformation is necessary. This additional information is for instancethe absolute main point data and/or absolute auxiliary point data and/orthe pelvis data. The relative point data are in particular incomplete inthat at least the position of one main point and at least the positionof one auxiliary point are necessary in addition to the relative pointdata, in order to calculate a position of the main plane. Thus, therelative point data can directly constrain the possible positions of themain plane relative to the at least one auxiliary point by describing apositional relationship (for example a distance or angle) between the atleast one auxiliary point and the main plane. The relative point datacan also indirectly constrain the positions of the main plane relativeto the at least one auxiliary point by describing a positionalrelationship (for example a distance or angle) between at least oneauxiliary point and at least one virtual auxiliary point.

In particular, the relative point data can include but do not need toinclude geometrical constraints which describe a predeterminedpositional relationship between planes of the anatomical pelvis, inparticular between the auxiliary plane and the main plane and/or betweenthe main plane and the standard plane and/or between the standard planeand the auxiliary plane. The relative point data in particular do notdescribe that this predetermined positional relationship is aperpendicular relationship. Nevertheless, the constraints allow thenumber of possible positions to be restricted. The relative point datacomprise in particular one scalar value, in particular only one scalarvalue, if the absolute point data comprise two main points. The relativepoint data also in particular comprise two scalar values, in particularonly two scalar values, if only one main point is provided by theabsolute point data. Examples of scalar values are distances and angles.

The relative point data are in particular stored in a data storage (e.g.RAM, ROM, or any database). The relative point data can be generated,preferably before the surgery starts, in particular outside theoperating theatre. The relative point data are in particular based on atleast one x-ray image of the pelvis. The relative point data are inparticular only based on two-dimensional x-ray images. In particular,the relative point data describe constraints which can be derived from(for example based on) a two-dimensional image, in particular only onetwo-dimensional image (but also possibly two or three or more), forexample only one x-ray image. Thus, the relative point data inparticular describe positional relationships between geometricalobjects, wherein said relationships are present in two dimensions (forexample in an image plane). In this way, the data processing method ofthe present invention can use data which are easily obtained and inparticular available before surgery. Thus, it is not necessary to takeadditional x-ray images, in particular during surgery. It is aparticular aspect of the invention that data available before surgeryare used to reduce the workload and so aid the surgeon when gatheringdata from the pelvis by means of a pointer. Furthermore, uncertaintiesin the position of the determined main plane due to a change in thelocation of the patient (due to being turned over from the supineposition to the lateral position) can be avoided. A specific example ofrelative point data is the (shortest) distance from an auxiliary pointto the main plane. Another example is where there are two auxiliarypoints which define a line. The relative point data can then describethe distance from one of the auxiliary points to the main plane whenfollowing the line and/or can define the angle of the line relative tothe main plane. The relative point data may also be based on anatomicalknowledge, for instance based on generic or statistical models of thepelvis. Based on these models, for instance distances or angels arederived which are used as relative point data. Furthermore, it is alsopossible to use, in particular before surgery, a mechanical tool inorder to measure the relative point data. For instance, the distancebetween the two ASIS points may be determined by using a mechanical tooland then the resulting distance may be entered into the system asrelative point data. The relative point data can not just describeconstraints for the possible positions of the main plane relative to theat least one actual auxiliary point but can in addition describeconstraints for the possible positions of the main plane relative to atleast one virtual auxiliary point.

As mentioned above, only a minimum number of auxiliary points,preferably but not obligatory in combination with an actual main pointcan be used as basis for the calculation of the position of the mainplane. Thus, there is a minimum of information needed for calculatingthe position of the main plane. However, of course, more than thisminimum information can be used as a basis for the calculation. Inparticular several actual main points or several actual auxiliary pointscan represent the basis for the calculation. In that case, a possibleerror in the calculation of the position of the main plane may bereduced by using this additional information in accordance with generalknown error reduction methods in case of more information as necessaryis available.

The step of calculating the position of the main plane relative to themarker device preferably includes a step of determining a virtualauxiliary point and/or a virtual main point as described above. Theposition of the main plane is then determined on the basis of thedetermined virtual main point and/or virtual auxiliary point. There isthus preferably an intermediate step of determining at least one virtualauxiliary point and/or at least one virtual main point (see below).

The majority and in particular preferably all of the actual auxiliarypoints used for calculating the position of the main plane arepreferably outside the main plane but on the same side of the mainplane. This is particularly advantageous if the main plane divides thepelvis into parts of at least approximately the same size. This is forinstance the case with the mid-sagittal plane (MSP). In this way, it ispossible to ensure that the auxiliary points are easily accessible forthe surgeon and that the data provided for the data processing methodare reliable data, since they were easily and clearly accessible for thesurgeon. Another embodiment will be described below, in which theauxiliary points are provided on both sides of the main plane butpreferably close to the main plane.

Preferably, the absolute auxiliary point data are used to determine(calculate) at least one additional main point which lies in the mainplane and which is the above-mentioned virtual main point. A virtualmain point is not included in the absolute main point data, but does liein the main plane. A virtual main point is in particular not based onthe detection of a landmark by means of a pointer. If there are inparticular no actual main point or only one or only two actual mainpoints, then there is not enough information available to determine theposition of the main plane on the basis of the absolute main point data.Therefore, in accordance with one embodiment of the invention, aparticular auxiliary point is used to determine at least one virtualmain point. One or more or all of the one or more auxiliary points canbe a particular auxiliary point. For the calculation of the virtual mainpoints also already calculated virtual main points and/or actual mainpoints may be used.

The particular auxiliary point can also be used to determine (calculate)at least one additional auxiliary point which lies outside the mainplane and which is the above-mentioned virtual auxiliary point. Avirtual auxiliary point is not included in the absolute auxiliary pointdata. A virtual auxiliary point is in particular not based on thedetection of a landmark by means of a pointer. The virtual auxiliarypoint can have a defined positional relationship, in particular asymmetrical relationship with respect to the main plane and preferablyalso with respect to the particular auxiliary point. The virtualauxiliary point is for example a point which is symmetrical to theparticular auxiliary point with respect to the main plane. As mentionedabove, the position of the main plane can then be determined on thebasis of the particular auxiliary point and the corresponding(symmetrical) virtual auxiliary point. Thus, there is then enoughinformation available to determine the position of the main plane on thebasis of the virtual auxiliary point.

As mentioned above, the relative point data include at least one scalarvalue. This at least one scalar value describes a positionalrelationship between an auxiliary point and the main plane (forinstance, the (shortest) distance from the auxiliary point to the mainplane). The scalar value can also describe a positional relationshipbetween two auxiliary points, one of which is a virtual auxiliary point.This virtual auxiliary point is in particular a point at a positionwhich is symmetrical to the other (actual) auxiliary point with respectto the main plane. An example of this is the sinistral and dextralanterior superior iliac spine (ASIS) point. For instance, the dextralASIS point is the actual auxiliary point and the sinistral ASIS point isthe virtual auxiliary point. This situation applies in particular if thepatient is lying on their sinistral side.

The scalar value can in particular describe the distance between twofossa points of the acetabulum (i.e. the deepest point of theacetabulum). The relative point data describe this distance according toan embodiment of the present invention. One of the two fossa points isan actual auxiliary point and the other one is a virtual auxiliarypoint. Thus, the determination method can generate the absoluteauxiliary point data e.g. by detecting the position of one of the fossapoints by means of a pointer. The generated absolute auxiliary pointdata can then be received by the data processing method. The distancebetween the two fossa points which is called inter-fossa distance can bededuced from an x-ray image. The inventors of the present invention havefound that deducing the distance from an x-ray image is not obligatory.The inter-fossa distance within males and within females are ratherconstant. Therefore, a particular distance for a male or for a femalecan be assumed. This distance represents an example for relative pointdata described by a scalar value. Typical values for inter-fossadistance for males is 116 mm. Typical values for inter-fossa distancefor females is 125 mm. For Asian people the values are a bit lower (114mm inter-fossa for males and 122 mm for females). In particular aninter-fossa distance of 116 mm +/−7 mm for males and 125 mm +/−8 mm hasbeen determined by the inventors. In particular, aninter-teardrop-distance of 112 mm +/−6 mm for males and aninter-teardrop-distance of 121 mm +/−8 mm for females has beendetermined by the inventors. Thus, preferably, the predetermined valuefor inter-fossa distance for males is set to be higher than 109 mmand/or smaller 123 mm for males and/or set to be higher than 118 mmand/or lower than 133 mm for females. In particular, the predeterminedvalue for an inter-teardrop-distance is set to be higher than 106 mmand/or lower than 118 mm for males and/or higher than 113 mm and/orlower than 129 mm for females.

Thus, according to an advantageous embodiment of the present invention,the inter-fossa distance is a predetermined value which is providedaccording to a step of the data processing method, in particularreceived by the data processing method as relative point data (e.g. froma database). In particular, this distance can be stored in a databaseand can be received from the data processing method by accessing thedatabase. Besides the inter-fossa distance, the inventors of the presentinvention have also found that the inter-teardrop distance is ratherconstant. Therefore according to another advantageous embodiment, theinter-teardrop distance is used as relative point data. In particular,there is a fixed relationship between inter-teardrop distance andinter-fossa distance which can be used if the position of one of thefossa points is described by the absolute auxiliary point data. Inparticular, the teardrop distance is smaller than the inter-fossadistance by a predefined difference which difference is in particularlarger than 1 or 2 mm and smaller than 3 or 4 mm. In particular, thedifference is about 2 to 3 mm. The inter-teardrop distance is generallyused by a physician. The physician determines the inter-teardropdistance by measuring a distance in an x-ray image of the patient.Preferably, the data processing method includes a step of providing thedifference between the inter-teardrop distance and the inter-fossadistance so that one of the two distances can be calculated if the otherone is provided (in particular received by the data processing method).For instance the physician measures the inter-teardrop distance from ax-ray image of the patient and inputs the distance into the dataprocessing method. Thus the inter-teardrop distance is received by thedata processing method and the inter-fossa distance can be calculatedfrom the inter-teardrop distance based on the aforementioned knowndifference. If no measurement of the distance is performed, according toan aspect of the invention, a predetermined value for the inter-teardropdistance and/or inter-fossa distance is used by the data processingmethod, in particular typical value is used by the data processingmethod. In other words, the data processing method includes the step ofproviding a predetermined value for the inter-teardrop distance and/orinter-fossa distance according to an embodiment.

Generally, according to an advantageous aspect of the invention,properties, in particular distances and angles of the anatomical bodypart (in particular the pelvis) are used for defining the relative pointdata which properties are at least approximately constant for particulartypes of human beings (in particular for particular types of racialpopulations (e.g. Asian, Caucasian or African) and/or for a particulartype of gender (male or female). Preferably those properties are usedwhich are describable by a scalar value (e.g. distance or angle) andwhich are at least approximately constant, i.e. the variation of thescalar value is low or zero. Low variation in the meaning of the presentinvention is in particular if the standard deviation from a mean scalarvalue is lower than 5%, 2% or 1% of the scalar value which describes theproperty. More preferably, low variation means that the variation of thescalar value results in a variation of less than 2 degree for theposition (orientation) of the acetabulum and/or femural shaft. Theposition (in particular orientation) is preferably described by an anglewith respect to a plane, in particular standard plane (see below).Preferably, a variation of the scalar value is considered to be a lowvariation, if the variation of the scalar value results in a variationof the angle of less than 5 degrees, in particular less than 2 degrees,in particular with respect to a target position (in particular targetorientation) which is in particular also described by an angle withrespect to a plane (in particular the standard plane). By referring togeneral properties which are approximately constant at least fordifferent types of human beings, further analysis of the anatomical bodyparts by means of analytical devices (like x-ray or CT or MRT) can beavoided and costs and time can be spared. Furthermore, radiation to thepatient and to the medical team can be reduced.

According to a further advantageous embodiment, the aforementionedproperties of the anatomical body part which are at least approximatelyconstant for particular types of human beings are described to be afunction of other properties of the human being. For instance, theinter-fossa distance or the inter-teardrop distance can be described tobe a function of the length of the femur or the body height. In thisway, it can be possible to determine the data used as relative pointdata more exactly and more individually by varying an approximatelyconstant anatomical property (in particular described by a scalar value)in accordance with the function.

Where the provision of an “auxiliary point” is mentioned here, it ismeant that an actual auxiliary point is provided unless otherwisespecified, i.e. an auxiliary point included in the absolute auxiliarypoint data. In all other cases it may be both (virtual and actualauxiliary point). Where the provision of a “main point” is mentionedhere, it is meant that an actual main point is provided unless otherwisespecified, i.e. a main point included in the absolute main point data.In all other cases, it may be both (virtual and actual main point). Inparticular, the absolute main point data describe only those positionsof main points which are used to calculate the position of the mainplane. In particular, the absolute auxiliary point data describe onlythose positions of auxiliary points which are used to calculate theposition of the main plane. As the actual main and/or auxiliary pointspreferably do, the virtual main and/or auxiliary points in particular(but not necessarily) represent landmarks. In particular, as far asherein the determination (calculation) of a position of a plane isconcerned and a reference is made to main points, if not otherwisespecified, this means preferably but not obligatory that not only actualmain points can be used for the determination (calculation) but alsovirtual main points can be used (in addition or exclusively). The sameapplies if a positional relationship between a main point and any othergeometric object (e.g. point or plane) is concerned.

In particular, as far as herein the determination (calculation) of aplane is concerned and a reference is made to auxiliary points, if nototherwise specified or clear from the description, this means preferablybut not obligatory that not only actual auxiliary points can be used forthe determination (calculation) but also virtual auxiliary points. Thesame applies if a positional relationship between an auxiliary point andanother geometric object (e.g. point or plane) is concerned.

Preferably, the position of the actual auxiliary point in combinationwith the relative point data allows the position of the virtualauxiliary point to be determined. If the virtual auxiliary point issymmetrical to the main plane with respect to the actual auxiliarypoint, determining the position of the virtual auxiliary point allowsthe position of the main plane to be determined.

Also the virtual auxiliary points may be determined indirectly based onthe detection of points outside the body part. For instance, the planeon which the patient is lying is determined by using a pointer. Assumingfurther, the patient is lying in lateral position, e.g. its sinistralASIS point is in contact with the plane. Furthermore, assuming, therelative point data describe the distance between the sinistral and thedextral ASIS point, then the determined location of the plane on whichthe patient is lying, allows to calculate, based on the relative pointdata and the position of the dextral ASIS point (which is provided bythe absolute auxiliary point data), the position of the virtualsinistral ASIS point. This virtual auxiliary point may be used for thecalculation of the planes in the same manner as the aforementionedactual auxiliary points.

The present invention also preferably uses anatomical knowledgeconcerning the pelvis. For example, as mentioned above, the relativepoint data can use this knowledge when referring to statistical modelsof the pelvis in order to determine distances or angles. For example,the above-mentioned inter-fossa or inter-teardrop distance representanatomical knowledge (found by the inventors) based on which therelative point data are provided.

Furthermore, the anatomical knowledge is preferably used to provide bodypart data (also referred to as pelvis data). These body part data(pelvis data) constrain the possible relative (anatomical) positionbetween landmarks of the pelvis and/or between the landmarks and themain plane. For instance, body part data (pelvis data) describe that oneof the landmarks is more anterior or more posterior or more distal ormore proximal or more cranial or more caudal than another landmark. Thebody part data (pelvis data) can also constrain the possible relative(anatomical) positions between (prominent) anatomical positions of(prominent) anatomical structures such as a crest or an anatomical plane(for example the mid-sagittal plane) or between such structures andlandmarks. Thus, the body part data can also be defined in the form ofinequality constraints.

In order to use the pelvis data provided, landmark data are preferablyprovided which link at least some of the main points and/or auxiliarypoints to the landmarks of the pelvis. The term “at least some” heremeans in particular at least two or at least three of the points, whichcan be main points and/or auxiliary points. In other words, the landmarkdata inform the navigation system as to which (actual or virtual) (mainor auxiliary) point represents which landmark. This information is givenfor at least some of the points.

It is possible, when attempting to determine the position of the mainplane on the basis of the provided absolute main point data, theprovided auxiliary point data and the provided relative point data, forthis attempt to result in more than one possible solution. If more thanone solution does result, the provided pelvis data and the providedlandmark data are in particular used to discount one or more of thesolutions which are not in line with the pelvis data, i.e. which do notcorrespond to the anatomy of the pelvis and can therefore be discounted.This allows one of the possible solutions for the position of the mainplane to be selected on the basis of the provided pelvis data and theprovided landmark data. In other words, the solution which is in linewith the constraints given by the pelvis data is selected.

As mentioned above, it is not obligatory to provide absolute main pointdata. However, in that case, preferably at least one virtual main pointis calculated based on the absolute auxiliary point data. Thiscalculated at least one virtual main point may be used in the furthercalculation in the same way as described below for the one or two actualmain points. In other words, the provision of actual main points can bereplaced by the calculation of virtual main points.

According to a particular embodiment of the present invention, theabsolute main point data describe a position of only one main point ofthe pelvis relative to the marker device or only one virtual main pointhas been calculated or can be calculated, then two (actual or virtual)main points are in particular missing in order to determine the mainplane since only one (actual or virtual) main point is available for thecalculation. In this case, the absolute auxiliary point data inparticular describe the position of at least two auxiliary points of thepelvis relative to the marker device. As mentioned above, the auxiliarypoints are outside the main plane. In particular, the actual auxiliarypoints are on the same side of the main plane. In this way, the dataprocessing method can be based on data which can be easily obtained by asurgeon. In particular, the absolute auxiliary point data describe theposition of only two auxiliary points.

In accordance with another embodiment, the absolute main point datadescribe the position of only two main points of the pelvis relative tothe marker device or two virtual main points have been calculated (orcan be calculated) or there is one actual main point and one virtualmain point, then one additional (virtual or actual) main point is inparticular missing in order to determine the main plane since only two(actual or virtual) main points are available for the calculation. Inthis case, the absolute auxiliary point data preferably describe theposition of at least one auxiliary point of the pelvis relative to themarker device, in particular the position of only one auxiliary point ofthe pelvis relative to the marker device. The auxiliary points are inparticular outside the main plane and in particular on the same side ofthe main plane. Thus, the data processing method can in this case againbe performed on the basis of data which are easily generated.

In accordance with another embodiment, the relative position datadescribe at least one constraint, in particular only one constraint forthe possible positions of the main plane relative to the at least oneauxiliary point, if the absolute main point data describe the positionof two main points, in particular of only two main points. In this case,there is in particular only one auxiliary point.

In accordance with another embodiment, the relative position datadescribe at least two constraints for the possible positions of the mainplane relative to the at least one auxiliary point, if the absolute mainpoint data describe the position of only one main point. In this case,the absolute auxiliary point data describe the position of at least twoauxiliary points, in particular of only two auxiliary points.

As mentioned above, the main plane is the mid-sagittal plane inaccordance with a preferred embodiment.

In the field of navigated surgery, in particular computer-assistedsurgery or image-guided surgery, a reference system (for example acoordinate system) in which the pelvis is located is preferablydetermined. In order to determine such a coordinate system, two planesdefined by the shape of the pelvis are preferably determined. One ofthese planes is in particular the mid-sagittal plane; the other planecan be the anterior pelvic plane or—as will be explained in more detailbelow—a spinae joint center plane as a new reference plane referred toas “SJCP”. The spinae joint center plane is defined by auxiliary points.In summary, there are preferably at least two planes which have to bedetermined in order to have an adequate basis for providing navigationinformation to the surgeon. These two planes and the resulting referencesystem are in particular used to define the position and in particularthe orientation of the acetabulum, in particular for hip surgery.

The present invention also relates to a data processing method whichincludes the above-mentioned data processing method and not only allowsthe position of the main plane of the pelvis but also an auxiliary planeof the pelvis to be determined.

The data processing method for determining the main plane and theauxiliary plane preferably uses the above-mentioned data processingmethod in order to determine the main plane. The above-mentioned dataprocessing method will therefore be referred to as the first dataprocessing method. The data processing method for determining both themain plane and the auxiliary plane will be referred to as the seconddata processing method. The second data processing method uses thefollowing approach for determining the auxiliary plane. Absoluteauxiliary point data are provided which describe the position of atleast two of the auxiliary points of the pelvis relative to the markerdevice. Thus, contrary to the first data processing method, there ispreferably a minimum of two auxiliary points, the position of which isknown from the absolute auxiliary point data. The auxiliary points arepreferably outside the main plane. The auxiliary points are inparticular on the same side of the main plane. Preferably, at least oneof the at least two auxiliary points is used for calculating theposition of the main plane. In accordance with another embodiment, theat least two auxiliary points are used for calculating the position ofthe main plane. Data are also provided which are referred to as relativeauxiliary plane data. These auxiliary plane data describe the positionalrelationship between the auxiliary plane and the main plane.Furthermore, it is assumed that the auxiliary points lie within theauxiliary plane.

In accordance with the second data processing method, the providedrelative auxiliary plane data are preferably used to calculate theposition of the auxiliary plane on the basis of the assumption that theat least two auxiliary points lie within the auxiliary plane. In otherwords, the auxiliary plane is determined in such a way that it includesthe at least two auxiliary points and fulfils the predeterminedpositional relationship with respect to the main plane, saidrelationship being known from the relative auxiliary plane data.

One example of the predetermined positional relationship is a particularangle between the main plane and the auxiliary plane. The angle can inparticular be within the range of 30° to 150°. The predeterminedpositional relationship can in particular be such that the auxiliaryplane is perpendicular to the main plane.

The invention is also directed to a third data processing method whichcomprises the second data processing method explained above. The thirddata processing method is a method for determining the position of themain plane and of a standard plane, wherein the position of theauxiliary plane has been determined by the second data processingmethod. The auxiliary points lying in the auxiliary plane are inparticular landmarks of the pelvis. Examples of auxiliary points will begiven below.

In accordance with the third data processing method, relative standardplane data are also provided. The relative standard plane data describethe expected relative positional relationship between the auxiliaryplane and the standard plane. The expected relative positionalrelationship is a positional relationship which can correspond to anaverage positional relationship derived from a statistical analysis ofthe positional relationship between the auxiliary plane and the standardplane for a plurality of different pelvises. In particular, the averagepositional relationship which results from this statistical analysisrepresents the expected positional relationship. Any kind of(statistical) method for determining an average can be applied, forinstance the arithmetic mean or the median or the mode. A generic modelof the pelvis can also be used to determine the relative positionalrelationship between the auxiliary plane of the generic model and thestandard plane of the generic model. This positional relationship alsorepresents an example of an expected positional relationship.

In accordance with the third data processing method, the position of thestandard plane is determined on the basis of the position of theauxiliary plane (determined using the second data processing method) andon the basis of the relative standard plane data. For instance, therelative standard plane data describe an angle between the position ofthe auxiliary plane and the position of the standard plane. Thus, thedetermined position of the auxiliary plane allows the position of thestandard plane to be calculated on the basis of the standard plane data.A standard plane is in particular a plane with respect to which theposition of the femoral shaft and/or the position (in particularorientation) of the acetabulum is (usually) described.

As mentioned above, the auxiliary points can represent landmarks in anyof the data processing methods described above (i.e. the first, secondand/or third data processing method). Examples of landmarks representedby the auxiliary points include the sinistral ASIS point or dextral ASISpoint. The auxiliary points can alternatively or additionally representa landmark which is defined by the acetabulum and/or by a partial(surface) point of the acetabulum. The auxiliary point can then inparticular be the rotational center or the deepest part of theacetabulum or the most proximal part of the acetabulum. The main planein any of the data processing methods described above (i.e. the first,second or third data processing methods) can for example be themid-sagittal plane. The standard plane as described with respect to thethird data processing method can for example be the anterior pelvicplane. The relative standard plane data described in connection with thethird data processing method describe in particular the angle betweenthe anterior pelvic plane and the auxiliary plane. The auxiliary plane(for example the SJCP) is in particular a plane described by thesinistral and dextral ASIS point and an auxiliary point defined by theacetabulum or a point or part of the acetabulum. The auxiliary plane isin particular such that the auxiliary plane and the anterior pelvicplane intersect each other along a line connecting the sinistral and thedextral anterior superior iliac spine landmarks of the pelvis. Thepresent application is also directed to an independent invention whichis directed to a data processing method which relies on the relativestandard plane data which describe the relative position between theanterior pelvic plane (APP) and the spinae joint center plane (SJCP).Thus, preferably any data processing method or system or computer etc.is a subject-matter of the present invention which relies on this kindof standard plane data. The inventors of the present invention havefirst found that the APP and SJCP have a statistically stable relativepositional relationship which may be used in any kind of data processingmethods which processes data related to the pelvis. Therefore, thepresent application is also directed to such an independent inventionwhich may be a subject-matter of a later divisional application. Thesedata processing methods are in particular directed to a calculation ofpositions, in particular orientations of planes or parts or points ofthe pelvis for computer aided navigation in surgery (image guidedsurgery). Such a data processing method is in particular as follows: Adata processing method for determining the position of a plane or a partor a point of pelvis which comprise the steps of: providing relativestandard plane data which describe the expected positional relationshipbetween the SJCP and the APP; and determining the position of the APP onthe basis of the position of the SJCP and the relative standard planedata.

The following embodiment can be combined with the aforementioned methodsor can also represent an independent embodiment of the invention. Inaccordance with this embodiment, additional auxiliary points aredetermined which are in particular symmetrical to another auxiliarypoint if the main plane is taken as the plane of symmetry. Said otherauxiliary point (which corresponds to the additional auxiliary point) isreferred to as the symmetrical auxiliary point. The (at least one)symmetrical auxiliary point is included in the absolute auxiliary pointdata, whereas the (at least one) additional auxiliary point is notincluded. As an independent embodiment of the invention, the (at leastone) symmetrical auxiliary point is provided in a step of theindependent data processing method. In accordance with this embodiment,relative auxiliary point data are provided. These relative auxiliarypoint data describe at least one constraint for possible positions ofthe additional auxiliary point relative to its corresponding symmetricalauxiliary point. When the symmetrical auxiliary point is on one side ofthe main plane, the corresponding additional auxiliary point is inparticular on the other side. In accordance with this embodiment,candidate point data are provided. The candidate point data describe theposition of a candidate point with respect to the marker device. Thesecandidate points are candidates for the additional auxiliary point. Newcandidate point data which describe the position of new candidate pointsrelative to the marker device are preferably provided in steps. All thecandidate points provided are preferably on the same side of the mainplane. The new candidate point data are preferably received in steps,such that candidate point data are provided in steps. A candidate pointreceived in the current step is referred to as the current candidatepoint. In accordance with this embodiment, the next step consists ofchecking whether the position of the current candidate point (i.e. thenew candidate point) complies with the at least one constraint. The atleast one constraint is described by the relative auxiliary point dataas mentioned above. An example of the constraint is in particular adistance between the symmetrical auxiliary point and its correspondingadditional auxiliary point.

After the checking step, the current candidate point (new candidatepoint) is preferably accepted as an additional auxiliary point if theposition of the current candidate point complies with the at least oneconstraint defined by the relative auxiliary point data, i.e. if theposition of the current candidate point is a possible position. If theposition of the current candidate point does not comply with the atleast one constraint, then the aforementioned steps (of providing andchecking) are repeated. Since the additional auxiliary point issymmetrical to the symmetrical auxiliary point with respect to the mainplane, the position of the main plane can be determined on the basis ofthe additional auxiliary point and the symmetrical auxiliary point. Thenew candidate point data are preferably generated by detecting a pointwhich contacts an additional anatomical structure which is symmetricalto another anatomical structure (referred to as the symmetricalstructure). This structure is in particular a prominent structure (forexample a rim or crest). A point detected on the symmetrical structurecorresponds to the symmetrical auxiliary point. A plurality of pointsdetected on the additional anatomical structure are candidate points forthe additional auxiliary point. During the movement of the pointer alongthe additional anatomical structure, new candidate points are generatedin steps. In order to guide the user during the movement of the pointer,indication data are preferably provided. Both the additional auxiliarypoint and the symmetrical auxiliary point are preferably determined onthe basis of the above-mentioned detection points (representing detectedlandmarks) and in particular not on virtual auxiliary points.

In accordance with another embodiment, the aforementioned methodcomprises additional steps which relate to generating indication data.These indication data are in particular based on the result of thechecking step and indicate whether the position of the candidate pointcomplies with the at least one constraint. These indication data arepreferably used by an indication method which comprises theaforementioned data processing method and which generates indicationsignals on the basis of the indication data in order to inform a user(surgeon) as to whether the current candidate point complies with theconstraint. Preferably, a deviation between a scalar value describingthe constraint (for instance a distance) and a scalar value describingthe position of the current candidate point relative to a main point orthe particular auxiliary point is calculated. The result of thecalculation is then preferably part of the indication data and is thusalso indicated to the user as part of the indication signal. A surgeoncan use the above-mentioned data processing method as an assistance infinding an auxiliary point when scanning the pelvis with a pointer.

The present invention is also directed to a program which, when it isrunning on a computer or is loaded onto a computer, causes the computerto perform the data processing method as described in any one of thepreceding embodiments. The present invention is also directed to aprogram storage medium (for instance a CD, ROM, RAM, harddrive, etc.) onwhich the program is stored. The present invention also relates to acomputer on which the program is running, by which the program isexecuted or on which the program is loaded. This computer in particularcomprises a memory which stores the program. The present invention isalso directed to a signal wave, in particular a digital signal wave,which carries the information represented by the program. A signal wavecarries the program for example during a download process whendownloading the program via the internet.

The present invention is also directed to a navigation system forcomputer-assisted surgery. This navigation system preferably comprisesthe aforementioned computer for processing the data provided inaccordance with the data processing method as described in any one ofthe preceding embodiments. Preferably, the navigation system comprises adetection device for detecting the position of the detection pointswhich represent the main points and auxiliary points, in order togenerate detection signals and to supply the detection signals generatedto the computer such that the computer can determine the absolute mainpoint data and absolute auxiliary point data on the basis of thedetection signals received. In this way, the absolute point data can beprovided to the computer. The navigation system also preferablycomprises a user interface for receiving the calculation results fromthe computer (for example the position of the main plane, the positionof the auxiliary plane and/or the position of the standard plane). Theuser interface provides the received data to the user as information.Examples of a user interface are a monitor or a loudspeaker. The userinterface can use any kind of indication signal (for example a visualsignal, an audio signal and/or a vibration signal).

Where data are described here as being “provided”, this means that theyare ready for use by the method in accordance with the invention. Thedata can achieve this state of being “provided” by for example beingdetected or captured (for example by a detection device) or by beinginputted (for example via interfaces) or by being determined on thebasis of input signals or detection signals or input data. The data canalso have this state by being stored in a memory (for example a ROM, CDand/or hard drive) and thus ready for use within the framework of themethod in accordance with the invention.

The expression “providing data” encompasses (within the framework of adata processing method) in particular that the data are determined bythe data processing method or program. The meaning of “providing data”in particular encompasses also that the data are received by the dataprocessing method or program (e.g. from another program or a datastorage), in particular to further process the data by the dataprocessing method or program. Thus “providing data” can mean forinstance to wait for a reception of data and to receive the data. Thereceived data can be for instance inputted by the interface. “Providingdata” can also mean that the data processing method or program performssteps to (actively) acquire the data from a data source, for instance adata storage (for instance ROM, RAM, data base, hard disk etc.) or viathe interface (for instance from another computer or a network). Thedata can achieve the state of being “ready for use” by performing afurther step before the providing step. According to the further step,the data are generated for providing the data. In particular the dataare detected or captured (for example by an analytical device).Alternatively or additionally, the data are input according to thefurther step, for instance via interfaces. In particular, the generateddata can be input (for instance in the computer). According to thefurther step (before the providing step), the data can also be providedby performing the further step of storing the data in a data storage(for example a ROM, RAM, CD and/or hard drive) and thus ready for usewithin the framework of the method or program in accordance with theinvention. The step of providing in particular does not encompass orcomprise an invasive step representing a substantial physicalintervention on the body which requires professional medical expertiseto be carried out and/or which entails substantial health risk even whencarried out with the required professional care and expertise. The stepof providing does in particular not comprise or encompass a surgicalstep. The step of providing does in particular not comprise or encompassa step for treatment of a human or animal body by surgery or therapy.The same applies for any steps directed to the determination of data.

In order to detect points on a plane, for instance actual main pointsand/or actual auxiliary points, a tool called plane point detection toolcan be used. The present invention is also directed to the use of suchplane point detection tools in combination with the above describedmethod and navigation system.

In particular, an independent aspect of the present invention isdirected to such a tool called plane point detection tool. The planepoint detection tool (also just called “tool” in the following) is fordetecting the position of at least one actual main point and/or at leastone actual auxiliary point. The plane point detection tool is preferablyconstituted to be arranged in a plane of the anatomical body part (e.g.main plane or auxiliary plane).

The present application is in particular directed to the independentinvention of the plane point detection tool which can be claimed inparticular as follows:

Plane point detection tool comprising

a proximal side constituted to be fitted to a patient by contacting thepatient along a first direction of length-wise extension of the tool,the proximal side is constituted to be able to be aligned with a planeof an anatomical body part along the first direction of length-wiseextension, the direction in which the proximal side faces and alongwhich the tool is flexibly bendable for bringing the tool into contactwith the patient being a third direction of extension of the tool;

a distal side which comprises at least one pointer insertion membercomprising a tapered recess for inserting the tip of a pointer, the atleast one pointer insertion member being prominently positioned in afirst section of the distal side which is, in the direction oflength-wise extension, closer to a first end of the plane pointdetection tool than to a second end;

wherein the distal side further comprises a second section having a flatsurface and being closer to the second end than to the first end;

both the proximal side and distal side extending in the first directionand in a second direction of extension, the second direction beingtraverse to the first direction,

the tool being flexibly bendable in the third direction while beingrigid in the second direction, the second direction being a direction ofwidth-wise extension of the tool, the width-wise extension of the toolbeing smaller than the length-wise extension.

The aforementioned tool can in particular be part of the navigationsystem of the present invention and can in particular be used incombination with the method, in particular the data processing method ofthe present invention. In particular, the tool is used for generatingdetection signals by means of a pointer which contacts the bottom of thetapered recess of the plane point detection tool. The tapered recess isin particular constituted such that the pointer can be pivoted duringthat contact.

The plane point detection tool preferably comprises a front side and aback side. The front side is close to the user (e.g. member of themedical staff, in particular surgeon) who inserts the pointer into theat least one tapered recess of the tool. The backside of the tool is tobe attached to the patient and is therefore called proximal side.Correspondingly, the front side is called distal side. According to apreferred embodiment for using the tool, the tool is attached to thebackside of the pelvis in the region of the sacrum along the midsagittalplane. That is, the direction of longitudinal extension of the tool isaligned with the midsagittal plane. Preferably, the at least one pointerinsertion member of the distal side is arranged at one end or close toone end of the longitudinal extension of the tool. In particular, inlongitudinal extension of the tool, the pointer insertion members arepositioned in one section of the distal side (called “first section”)while no pointer members are present in the reminder of the distal side(called “right section”). In this way, the detection of points which arelying in a plane, in particular the midsagittal plane, and which are notaccessible can be replaced by detection of the position of the pointerinsertion members which are accessible since they are placed closed toone end (called first end) of the tool (in longitudinal direction), inparticular within one half (called first half) of the tool (in the firstsection). In particular the second section is within the other half ofthe tool or comprises the other half of the tool (called 2^(nd) half).Thus, the at least one pointer insertion member is (in longitudinaldirection) closer to the first end of the tool than to the other end(called second end) of the tool. Preferably, that one of the at leastone pointer insertion members which is closest to the second end has atleast a distance of 5 cm, 10 cm, 15 cm or 20 cm from the second end,preferably at least 15 cm. In this way, the pointer insertion members donot interfere with fixing devices which are used to fix the anatomicalbody part in particular during surgery and which contact the rightsection but do not contact the left section.

Preferably, the plane point detection tool (shortly just called “tool”)comprises at least two pointer insertion members in order to detect twopoints in a plane of the anatomical body part. Preferably, the distancebetween that one of the two pointer insertion members which is closestto the first end and that one of the pointer insertion member which isclosest to the second end is at least 1 cm, 3 cm, 5 cm or 10 cm,preferably at least 3 cm. The greater the distance, the higher is theaccuracy of detection of a position, in particular orientation of aplane. Preferably, that one of the at least two pointer insertionmembers which is closest to the second end is (still) closer to thefirst end than to the second end. According to a further embodiment, twopointer insertion members have a distance of preferably at least 5 cm or10 cm or 15 cm (preferably at least 10 cm) and in particular allow apatient fixing device to be positioned in between. In particular in thatcase, that one of the at least two pointer insertion members which isclosest to the second end is closer to the second end than to the firstend.

Preferably, the distal side comprises a flat surface section, inparticular plane surface section which is closer to the second end thanto the first end. The length of these flat surface portion (inlongitudinal direction of the tool) is preferably longer than 5 cm, 10cm, 15 cm or 20 cm. Due to the flat surface section, pressing of afixing device to an anatomical body part is not obstructed. Inparticular, the flat surface section adopts at least a part of thesecond half of the distal side which is closest to the second end.

Preferably, the left section comprises a flat section facing in distaldirection. Preferably, the pointer insertion members are projecting fromthe flat surface section which is closer to the first end than to thesecond end. In this way, the pointer insertion members are palpable (canbe sensed by a member of the medical stuff) due to difference in heightof the flat section and the distal end of the prominent pointerinsertion member, even through a drape which covers the distal side ofthe tool. Thus, it is possible to insert a pointer tip into the pointerinsertion member although there is a drape between the pointer tip andthe tapered recess of the pointer insertion member. In this way, it ispossible that the drape can rest on the patient during detection of thepoints lying in the plane of the anatomical body part (in particularmidsagittal plane).

Preferably, the proximal side is constituted to be fitted to a patient.The proximal side of the tool is in particular fitted to a patient bycontacting the proximal side with the patient. The contact is inparticular along the first direction of longitudinal (length-wise)extension of the tool. Preferably, there are a plurality of contacts, inparticular a multiplicity (manifold) of contacts between the patient andthe tool in order to achieve the fitting. To this end, the proximal sidefor instance comprises an adhesive which is constituted to adhere thetool to the patient's body. According to another embodiment, theproximal side comprises a plurality of sucking portions (e.g. suckingcups) which allow to fit (in particular fix) the tool to the body.

Preferably, the plane point detection tool is flexible if flexed inproximal direction. This proximal direction is called third direction.The third direction is in particular normal to the surface of theproximal side. Since the tool is flexible in the third direction, thetool can be attached to a curved surface of the body part. Preferably,the tool is rigid in a second direction. In other words, the tool ispreferably not bendable in the second direction. The second direction ispreferably perpendicular to the third direction and to the firstdirection. The second direction is preferably a width-wise direction ofthe tool while the first direction is preferably a length-wise(longitudinal) direction of the tool. Preferably, the length of thelongitudinal extension of the tool is longer than the length of thewidth-wise extension of the tool. Preferably, the length-wise extensionof the tool is longer than three times, five times or ten times of thewidth-wise extension of the tool. Due to the longer extension of thetool in the lengthwise direction than in the width-wise direction anadaptation of the shape of the tool to the shape of the surface of thebody of the patient is possible although the tool is rigid in thewidthwise direction. Preferably, the width of the tool in width-wisedirection (at least with respect to the second section) is smaller than5 cm, 3 cm, 2 cm or 1 cm and/or larger than 1 mm, 2 mm or 5 mm. Thewidth of the tool at the first section can be wider than at the secondsection.

Preferably, the tool is constituted to be fitted to the body at leastwithin a part of the section which is opposite to the second section atthe proximal side.

In this way, the tool is attachable to the body at least in a section ofthe tool which is assumed to be obstructed by the aforementioned fixingdevice while the fixing device can be brought into contact with theanatomical body part and the second section without interfering of theplane point detection tool. In particular, the fixing device can exertpressure on the anatomical body part via the tool. Preferably, to thisend, the thickness of the plane point detection tool is smaller than thelength and in particular also smaller than the width of the plane pointdetection tool. Preferably, the thickness (i.e. the extension of thetool in the third direction) is preferably smaller than a half, inparticular smaller than a fifth, in particular smaller than a tenth ofthe width of the tool. The thickness of the tool (at least with respectto the second section) is preferably smaller than 1 cm, 5 mm, 2 mm or 1mm. The aforementioned definitions of the thickness of the tool apply inparticular for the second section of the tool which is close to thesecond end and which comprises or consists of a flat surface. The firstsection which includes at least one pointer insertion member can have amaximum thickness which is larger than the thickness as defined before.In particular, the thickness at the position of the pointer insertionmembers can be more than 1 mm or 2 mm or 5 mm or 1 cm thicker than thethickness of the second section (in particular due to the height of thepointer insertion member) and/or can be a half or equal to or twice thewidth of the plane point detection tool.

In the following detailed description, other features of the presentinvention are disclosed. The features of different embodiments can becombined.

FIG. 1 shows a navigation system in accordance with the invention;

FIG. 2 shows the use of main and auxiliary points for determining themid-sagittal plane;

FIG. 3 shows a plane point detection tool;

FIGS. 4 to 6 show situations of use of the plane point detection tool.

FIG. 1 schematically shows a pelvis 10. Attached to the pelvis 10 is areference star 20. There is a fixed spatial relationship between thereference star 20 and the pelvis 10. A surgeon can contact parts, inparticular landmarks, of the pelvis 10 by means of the tip of a pointer30. Attached to the pointer 30 are markers, in particular anotherreference star 40. The location of the reference stars 20 and 40 (andthe corresponding markers) can be detected by the detection device 50.The process of contacting the pelvis with the pointer and detecting themarkers of the pointer is also referred to here as “scanning”. Thedetection device 50 supplies the detection signals to the computer 60.The computer 60 includes a database which stores the relative spatialrelationship between the markers of the pointer and the tip of thepointer. In the computer 60, the absolute point data are then determinedon the basis of the detection signals received from the detection device50. The relative point and/or pelvis data can be input or can be storedin the database. Due to this determination, the absolute point data areavailable and thus provided.

A keyboard 70 and mouse 80 and a monitor 90 are for example connected tothe computer 60. The monitor 90 serves to display information to theuser such as the results of the processing performed in accordance witha data processing method of the invention and/or indication signals asalready mentioned above.

FIG. 2 shows the pelvis 10 in more detail. The attached reference star20 is not shown. FIG. 2 also shows a landmark TSP which is the ASISpoint. This landmark TSP has been scanned by the pointer 30, and thecorresponding position information has been supplied to the computer 60of the navigation system. The navigation system comprises the detectiondevice 50, the computer 60 and the monitor 90. Two landmarks MP1 and MP2have also been scanned by the pointer 30. These landmarks MP1 and MP2lie in the mid-sagittal plane 100. For instance, in the following thepoints MP 1 and MP2 are assumed to be points on the sacrum and lumbarspine. The point TSP is an example of an auxiliary point, and the pointsMP1 and MP2 are examples of main points. Another main point would be thepubic landmark 110. However, in the following, it is assumed that theabsolute main point data describe only the position of MP1 and MP2 andthat the absolute auxiliary point data describe only the position ofTSP. In the following, it will be described how the position of the mainplane (the mid-sagittal plane 100) is determined on the basis of onlytwo main points and only one auxiliary point.

In accordance with one possible procedure, a line 120 is determined byMP1 and MP2. A line 130 is drawn from the point TSP and intersects theline 120 at a right angle. Thus, the line 130 is perpendicular to theline 120. The intersection between the line 120 and the line 130 definesthe center of a circle 140. The circle 140 includes the point TSP andanother point NTSP. The point NTSP is a virtual auxiliary point (i.e.NTSP has not been scanned). The point NTSP is symmetrical to the pointTSP with respect to the mid-sagittal plane 100. In other words, if thepoint TSP is mirrored in the mid-sagittal plane 100, this would resultin the point NTSP. The point NTSP is the solution which is to be found.Relative point data are used to find the point NTSP. The relative pointdata describe the distance between NTSP and TSP. This distance can forexample be derived from an x-ray of the pelvis 10. The point NTSP has tobe on the circle 140 and has to fulfill the constraint, i.e. has to havethe predetermined distance from the point TSP. There are two possiblesolutions for these constraints, namely NTSP and the point 150. Thepoint 150 is an incorrect solution. The pelvis data are used to discountthe incorrect solution 150 from the two possible solutions. The pelvisdata describe in particular anatomical characteristics of the pelvis.These data describe in particular that the point NTSP is more anteriorthan the point MP1. The point 150, however, is more posterior than MP1.Therefore, this possible solution can be discounted. The solution NTSPis therefore selected, since this point complies with the anatomicalcharacteristic that the solution has to be more anterior than MP 1. Theconstraint that NTSP has to be more anterior than MP1 represents anexample of the possible relative positions between MP1 and NTSP. Thesepossible relative positions are constraints described by the pelvisdata. The circle 140 is incidentally also perpendicular to the line 120.

In accordance with another possible data processing method, a sphere isconstructed which has a radius corresponding to the distance between MP1and TSP. The center of this sphere is located at the point MP1. A secondsphere is also constructed which has a radius corresponding to thedistance between MP2 and TSP. The center of the second sphere is locatedat the point MP2. The intersection between these two spheres is thecircle 140 which corresponds to a multitude of possible solutions forthe location of the point NTSP. In the same way as mentioned above, thismultitude of possible solutions is firstly restricted to the point NTSPand the point 150 by using the constraint which defines the distancebetween NTSP and TSP. In a subsequent step, the pelvis data are thenused to discount the point 150 from the possible solutions, such thatthe point NTSP remains as the only possible solution.

As mentioned above, NTSP and TSP are symmetrical to each other withrespect to the mid-sagittal plane. Therefore, the mid-sagittal plane isdefined as being perpendicular to the line which connects NTSP and TSP,hence the mid-sagittal plane is determined.

In accordance with another preferred step, another plane—the auxiliaryplane—is determined, with the aim of defining a coordinate system inwhich the pelvis rests. For this purpose, another auxiliary point 160 isprovided. To this end, the landmark corresponding to the auxiliary point160 is preferably scanned by the pointer 30. The auxiliary plane is thendefined as being perpendicular to the mid-sagittal plane 100 andincluding the two auxiliary points TSP and 160. The auxiliary point 160is preferably based on the acetabulum. It can for example be therotational center of the femoral head in the acetabulum or the bottom ofthe acetabulum (fossa) or the center of the rim of the acetabulum, toname but a few examples. The inventors of the present application havefound that there is a reliable and fixed spatial relationship between astandard plane (APP) and the aforementioned auxiliary plane whichincludes a landmark defined by the acetabulum and the ASIS point asanother auxiliary point. The point TSP preferably corresponds to theASIS point, in which case the auxiliary plane has a defined positionalrelationship to the anterior pelvic plane (APP) which is often used as areference for defining the orientation of the acetabulum. This standardplane is also perpendicular to the mid-sagittal plane and has an angleof about 40° with respect to the aforementioned plane which includes theASIS point and the acetabulum landmark point. This auxiliary plane isalso referred to here as the spinae joint center plane (SJCP).

In the manner described above, at least two planes are defined whichallow a coordinate system to detected defined. The planes can forexample be the mid-sagittal plane and the auxiliary plane (in particularthe SJCP) and/or can be the mid-sagittal plane and the APP.

In the following, the data processing method is described for thescenario in which there is only one main point on the mid-sagittalplane. In this case, the relative point data preferably comprise twoconstraints or more specifically two scalar values. One scalar valuedescribes the distance between the point NTSP and the point TSP. Theother scalar value describes the distance between the acetabular orauxiliary point 160 and the symmetrical acetabular or auxiliary point160′. This distance is in particular the aforementioned inter-fossadistance. In other words, the symmetrical acetabulum 160′ is symmetricalwith respect to the mid-sagittal plane, i.e. the point 160′ is amirrored point of the acetabular point 160.

The two scalar values, i.e. the two distances, are in particular knownfrom x-ray images. The distance between the point TSP and the point 160is also known due to the absolute auxiliary point data. In a subsequentstep, a line can be constructed on the basis of the TSP point and thepoint 160. This line crosses the mid-sagittal plane 100. The distancebetween the point 160 and the mid-sagittal plane 100 along this line canbe calculated on the basis of geometric laws by using the aforementionedtwo distances between the point TSP and the point NTSP and between thepoint 160 and the point 160′. Thus, the position at which the line whichincludes the point TSP and the point 160 crosses the mid-sagittal planecan be calculated. Thus, a second point on the mid-sagittal plane isknown. The procedure already known from the above description can thenbe applied in order to determine the position of the point NTSP or thepoint 160′. If these positions are known, then the mid-sagittal plane isdefined as being perpendicular to the line connecting the point NTSP andthe point TSP and/or perpendicular to the line connecting the point 160′and the point 160. The SJCP or the APP can also then be calculated inthe manner described above.

The above-mentioned inter-teardrop distance can be measured in ananterior/posterior x-ray image of the pelvis. Within an extensiveanalysis of CT data sets it had be found by the inventors that theinter-teardrop distance as well as the inter-fossa distance are almostconstant across various populations. Between males and females there aresignificant differences. Thus, a gender-specific value for the distancesis preferably used as relative point data. For fine-tuning purposes, thevalues could also be adjusted to certain populations (e.g. Asian,European, US). In particular, a fixed value for the distance can beassumed for a specific population in order to provide the relative pointdata.

The inter-teardrop and inter-fossa distances are in particular definedas described in the following. The teardrop figure is a well-knownstructure which can be identified in anterior/posterior x-rays of thepelvis. Within the mentioned CT data analysis, artificially generatedx-ray images (i.e. digitally reconstructed radiographs-DRRs) were usedto reproduce the teardrop figure for evaluation purposes. The teardropfigure depicts a structure close to the medial wall of the acetabulumand can be determined on each side of the pelvis. In particular themedial-lateral position of the structure can be determined very well.The inter-teardrop distance is a distance between the teardrop figureson a medial-lateral line through the pelvis.

The fossa is a region within the acetabulum which is deeper than therest of the approximately spherical structure of the acetabulum. Itbasically lies on the medial side of the acetabulum. The inter-fossadistance is the distance between the fossa regions on both sides of thepelvis in a medial/lateral direction. The fossa region can be easilyidentified intra-operatively, e.g.

palpated with a pointer device and acquired with a navigation system.For evaluation purposes, the fossa region can be identified directly inthe CT datasets.

The fossa region is an anatomical structure which significantlyinfluences the position of the teardrop figure (see also Bowerman J W,Sena J M, Chang R: The teardrop shadow of the pelvis; anatomy andclinical significance. Radiology 1982 Jun; 143(3):659-62). Thus, bothdistance values (inter-fossa distance and inter-teardrop distance) arestrongly correlated. For practical purposes, the inter-teardrop and/orthe inter-fossa distance can be used.

Typical values for the distances are 112 mm (inter-teardrop) and 116 mm(inter-fossa) for males. For females, the standard values are 121 mm(inter-teardrop) and 125 mm (inter-fossa). For Asian people, the valuesare a bit lower (110 mm inter-teardrop and 113 mm inter-fossa for males,117 mm inter-teardrop and 119 mm inter-fossa for males). Additionally,other procedures to determine orientations of the pelvis or otherplanning parameters can be based on the proposed distances as well. Inparticular the orientation according to rotations around acranial/caudal axis can be determined. In particular information aboutthe anteversion of the acetabulum/cup implant e.g. for THR surgeries canbe determined.

Using the SJCP to define a coordinate system can be advantageous, sincethe APP might be less reliably determined by scanning, i.e. using apointer to detect the landmarks on the APP. The pubic landmark 110 inparticular can introduce uncertainties into the method due to differingamounts of fat and other soft tissue above the pubic area. These tissuescan prevent the position of the point 110 from being accurately detectedby means of a pointer.

The coordinate systems defined in accordance with the method of thepresent invention can be used for registration and also for other taskswithin planning and navigation steps. The method of the presentinvention (i.e. the data processing method) is in particular used forplanning surgery and for computer-assisted navigation. Because of thereliable and consistent relationship between a coordinate system definedby the mid-sagittal plane and SJCP and standard coordinate systems, allinformation can be transferred back and forth between the coordinatesystems.

In accordance with another embodiment, one main point is scanned by thepointer on the mid-sagittal plane. In other words, one main point isprovided to the data processing method. One additional (first) point ona structure, in particular a prominent structure, of the pelvis isdetermined as an auxiliary point (symmetrical auxiliary point). Anexample of the prominent structure is the posterior aspect of the iliaccrest. The data processing method of the present invention, performed inparticular on the navigation system of the invention, then assists theuser in finding another (second) auxiliary point by issuing theindication signal mentioned above. This second point is symmetrical tothe first point with respect to the mid-sagittal plane. For thispurpose, the navigation system shows additional information (indicationsignals) such as distance values or angles. The rationale behind thisapproach is that the user is assisted in finding the second point whichhas the same relationship (for example distance/angle) to themid-sagittal plane as the first point. The user can thus proceed alongthe prominent structure until the system informs the surgeon that thedesired relationship has been found. In general, this approach can beused to find new reference points (new auxiliary points) on ananatomical object from given points and the specific relationships(geometric features/constraints) between the given point and the pointsto be determined. The navigation system according to the inventionassists in finding the new points (new auxiliary points) by providinginformation concerning the geometric features/constraints. The user canthus find these new auxiliary points by locating them on a prominentstructure. This prominent structure can be close to the mid-sagittalplane.

The above-mentioned embodiment can in particular be used in the case oflateral pelvic registration. The embodiment can be used to find a secondpoint which lies symmetrical to the first point with respect to themid-sagittal plane. The line between these two points represents themedial-lateral direction. This can be used as an additional constraint.It provides not only a one-dimensional but a two-dimensional constraint.Thus, only one point on the mid-sagittal plane is required in this case.In the same way as in the previous embodiments, additional information(relative point data and/or pelvis data) can be used to make theregistration more robust.

FIGS. 3 a, 3 b and 3 c show an embodiment of a plane point detectiontool. FIG. 3 a is a perspective view of the embodiment. FIG. 3 b is aside view of the embodiment. FIG. 3 c is a top view of the embodiment.

FIG. 3 c shows the embodiment of the plane point detection tool from thebird's eye view (top view). The arrow A shows the direction oflongitudinal (length-wise) extension of the tool. The arrow B shows thedirection of width-wise extension of the tool. The width-wise extensionof the tool is for example 1 cm and preferably has an extension largerthan 3 mm and/or smaller than 4 cm. This upper end and/or lower limitapplies in particular for the section “f” (second section) which is thesection on the right side in FIG. 3 c and which extends from the secondend of the tool over a major part of the tool towards but not up to thefirst end. The remaining part of the tool is called section “p” (firstsection). This section p extends from the first end of the tool at leastuntil it includes that one of the pointer insertion elements which isclosest to the second end. According to an embodiment, the section p(first section) and section f (second section) are adjoining to eachother. Section f includes in particular a major part of the tool (inlongitudinal direction of the tool). In particular the section f ispreferably constituted to be fitted to a patient. To this end,preferably the distal side and/or the proximal side of the tool is flat,in particular plane. FIG. 3 c shows the distal side of the tool 200. Inparticular, two pointer insertion tools 210 and 220 are prominent, inparticular prominent from a flat portion 230 which preferably extendsfrom the first end to the second end. The flat portion 230 comprises inparticular opposite flat surfaces S1 and S2 (see FIG. 3 b). The surfaceS2 faces towards the distal side and the surface S1 faces towards theproximal side. The flat portion 230 has preferably a tape like shape. Inparticular the surface Si and S2 have a rectangular shape. Preferablythe extension of the flat portion in the direction A is longer than theextension in the width-wise direction B for more than a factor 2, inparticular 3, preferably 5. The flat portion 230 is preferably rigid inthe direction B while it can be bent in a direction C. That is, the flatportion 230 is preferably flexible in the direction C. That is, in theside view from a direction as shown in FIG. 3 b, the tool 200 can adopta curved or undulated shape as is indicated by the perspective viewshown in FIG. 3 a. Preferably, the tool is rigid in the direction B,i.e. cannot be bent. Rigid means in particular that the tool cannot bebent (e.g. by at least 1 cm in the direction B at the second end) by aforce applyable by human being which is in particular a force of morethan 10 N and less than 1000 N. The force is applied on one end of thetool (e.g. second end) while the other end (e.g. first end) is fixed.Preferably, such a force which results in a bending (e.g. by at least 1cm in the direction (e.g. at the second end) is smaller than 1000 N, inparticular smaller than 100 N or 10 N if the force is applied in thedirection C (at one end, e.g. 2^(nd) end of the tool while fixing theother end, e.g. first end of the tool).

Preferably, the pointer insertion elements 210 and 220 are attached tothe flat portion, for instance by gluing or by means of a fixing element(for instance a screw or a rivet). The pointer insertion tools havepreferably prominent outside walls 211 and 222 (see FIG. 3 b) which arepreferably projecting from the flat surface S2 into the distaldirection. Inside the walls there is preferably a recess, in particularpointed recess which allows to insert the tip 310 of a pointer 300 (seeFIG. 6). The recess has in particular conical shape and is preferablytapered from the distal end of the pointer insertion elements towardsthe second surface (i.e. in proximal direction).

Besides the rectangular shape shown in FIG. 3, the tool can for instancealso have oblong or elliptical shape.

FIG. 4 shows the tool 200 in use. The tool 200 is attached to the backof a patient is aligned with the midsagittal plane of the pelvis. Thealignment is possible by sensing the spinus process of the sacrum. Inparticular, the tool 200 is attached by means of an adhering layer whichcomprises the surface Si of the tool 200. In this way, the tool 200 canbe attached to the skin as shown in FIG. 4. In other words the tool 200has preferably the properties of a sticky tape, in particular isconstituted as a sticky tape. Since the tool is flexible in thedirection C, the tool can be smoothly (snugly) fitted to the curvedsurface of the patient's skin. Since the tool is small in width-wisedirection, the rigid property of the tool in the width-wise directiondoes not hinder the attachment of the tool to the curved skin.Additionally, the rigid property assures that the pointer insertionelements 210 and 220 are aligned in the same direction as thelongitudinal extension of the section f of the tool. Thus, if thelongitudinal extension of the section f is aligned with the midsagittalplane also a virtual line which connects the at least two pointers 210and 220 is aligned with the midsagittal plane. Thus the at least onepointer insertion element (210 and 220) are positioned in themidsagittal plane.

FIG. 5 shows the attachment of a patient fixing device 400 comprising acushion 410 and a support structure 420 for the cushion which is fixedto the couch on which the patient is lying. The cushion 410 is incontact with the back skin of the patient and the area of the pelvis andthus obstructs at least a part of the section f of the tool 200. Thefixing device is also called posterior support.

Thereafter, as shown in FIG. 6 the patient can be draped as usual. Inparticular a marker device (reference star 500) can be fixed to thepelvis of the patient. After fixing the reference star 500 it is ofadvantage to teach the navigation system the position of the plane (inparticular midsagittal plane) relative to the reference star 500 inorder to help the surgeon during surgery by means of computer assistedsurgery (navigation). Without the tool 200, the problem would be herethat the projecting parts of the sacrum cannot be sensed (palpated) orare difficult to be sensed by the surgeon or any other member of themedical staff. However, due to the tool 200, the pointer insertionelements 210 and 220 can be sensed (palpated) even through the drapesince the pointer insertion tools are prominent and their recesses aresensible. In particular, the dimension of the opening of the recess inthe pointer insertion member is preferably larger than 1 mm, 3 mm or 5mm and in particular smaller than 2 cm or 1 cm. Therefore, the operator(surgeon or any other member of the medical staff) is able to insert thetip 310 of the pointer 300 into the recess of the pointer insertionmember 210 and 220. In this way, the operator can teach to thenavigation system the positions of the tips of the recesses 210 and 220which are positioned in the midsagittal plane. In particular thedetection of the position of the pointer (by detecting the markers ofthe pointer) allows to determine the position of the tip of the pointer(due to data describing the relative position between the markers of thepointer and the tip of the pointer) and to thus generate absolute mainpoint data.

1. A data processing method for determining the position of a plane ofan anatomical body part called main plane, constituted to be performedby a computer and comprising the steps of: providing data calledabsolute auxiliary point data which describe the position of at leastone point of the body part, called actual auxiliary point, relative to amarker device attached to the body part, the at least one actualauxiliary point being outside the main plane; providing data calledrelative point data which constrain the possible positions of the mainplane relative to the at least one actual auxiliary point; providingdata called absolute main point data which describe the position of oneor two points of the body part, called actual main points, relative tothe marker device attached to the body part, said one or two actual mainpoints lying in the main plane and/or calculating the position of atleast one point called virtual main point relative to the marker device,said at least one virtual main point being in the main plane and beingcalculated based on the absolute auxiliary point data and the relativepoint data; calculating a position of the main plane relative to themarker device, wherein the calculation uses the relative point data andauxiliary point data as well as the provided absolute main point dataand/or the calculated position of the at least one virtual main point.2. The data processing method according to claim 1, wherein the step ofcalculating the position of the main plane includes the step ofdetermining at least one virtual auxiliary point and/or at least onevirtual main point on the basis of the relative point data and theabsolute main point data and/or absolute auxiliary point data, andwherein the step of calculating the position of the main plane alsoincludes the step of calculating the position of the main plane on thebasis of the at least one virtual auxiliary point and/or at least onevirtual main point, wherein the at least one virtual main point is notincluded in the absolute main point data and the at least one virtualauxiliary point is not included in the absolute auxiliary point data. 3.The data processing method according to claim 1, wherein the relativepoint data describe at least one scalar value used for describing apositional relationship between a particular auxiliary point of the oneor more actual auxiliary points and the virtual main point and/orbetween the particular auxiliary point and the virtual auxiliary point,the position of the virtual auxiliary point being in particularsymmetrical to the position of the particular auxiliary point withrespect to the main plane, and/or between one or more of the actualauxiliary points and the main plane.
 4. The data processing methodaccording to claim 1, further comprising the steps of: providing datacalled body part data which constrain the possible relative positionsbetween landmarks of the body part and/or between the landmarks and themain plane; providing data called landmark data which respectivelycorrelate at least some of the actual and/or virtual main points and/orauxiliary points with at least some of the landmarks of the body part;and if more than one solution for calculating the position of the mainplane is possible, selecting one of the possible solutions for theposition of the main plane on the basis of the landmark data and bodypart data.
 5. The data processing method according to claim 1, whereinall of the actual auxiliary points described by the absolute auxiliarypoint data and used for calculating the position of the main plane areoutside the main plane but on the same side of the main plane.
 6. Thedata processing method according to claim 1, wherein if the position ofonly one main point of the body part relative to the marker device isavailable for the calculation, then the absolute auxiliary point datadescribe the position of at least two auxiliary points of the body partrelative to the marker device, the at least two auxiliary points beingoutside the main plane; and if the positions of only two main points ofthe body part relative to the marker device are available for thecalculation, then the absolute auxiliary point data describe theposition of at least one auxiliary point of the body part relative tothe marker device, the at least one auxiliary point being outside themain plane.
 7. The data processing method according to claim 5, whereinthe relative point data describe at least one constraint for thepossible positions of the main plane relative to the at least oneauxiliary point, if there are two main points, and describe at least twoconstraints for the possible positions of the main plane relative to theat least one auxiliary point, if there is only one main point.
 8. A dataprocessing method which includes the data processing method of claim 1and which is a method for determining the position of the main plane ofthe anatomical body part and of another plane called auxiliary plane ofthe anatomical body part, wherein in order to determine the main plane,the data processing method of any one of the preceding claims isperformed, and in order to determine the auxiliary plane, absoluteauxiliary point data describe the position of at least two of theauxiliary points of the body part relative to the marker device, the atleast two auxiliary points being outside the main plane; furthercomprising the step of providing relative auxiliary plane data whichdescribe a predetermined positional relationship between the auxiliaryplane and the main plane; wherein the position of the auxiliary plane isdetermined by assuming that the at least two auxiliary points lie in theauxiliary plane, and is determined on the basis of the relativeauxiliary plane data.
 9. A data processing method which includes thedata processing method of claim 8 for determining the position of themain plane and of the auxiliary plane and which is additionally a methodfor determining the position of a standard plane which is different fromthe auxiliary plane, further comprising the steps of: providing relativestandard plane data which describe the expected positional relationshipbetween the auxiliary plane and the standard plane; and determining theposition of the standard plane on the basis of the position of theauxiliary plane and the relative standard plane data.
 10. The dataprocessing method according to claim 1, wherein: one of the landmarksrepresented by one of the auxiliary points is the sinistral or dextralanterior superior iliac spine landmark; and/or the main plane is themid-sagittal plane; and/or the standard plane is the anterior pelvicplane; and/or the standard plane data describe the angle between theanterior pelvic plane and the auxiliary plane; and/or the anteriorpelvic plane and the auxiliary plane intersect each other along a lineconnecting the sinistral and the dextral anterior superior iliac spinelandmarks of the body part; and/or another auxiliary point represents alandmark defined by the acetabulum or a part of the acetabulum or thefossa of the acetabulum or another point of the acetabulum; and/or therelative point data describe a constraint for an inter-fossa distance oran inter-teardrop distance of the pelvis.
 11. A data processing methodfor determining additional points called additional auxiliary pointswhich can be used for determining the position of a plane of a bodypart, the plane being called a main plane, in particular according toclaim 1, comprising the steps of: providing at least one auxiliarypoint; providing relative auxiliary point data which describe at leastone constraint which limits the possible positions for an additionalauxiliary point, the additional auxiliary point being symmetrical to oneof the one or more auxiliary points with respect to the main plane; andproviding candidate point data in steps, wherein in each step, theposition of a new candidate point is described by the candidate pointdata, wherein in the current step, the following steps are performed: a)providing new data called new candidate point data which describe arelative position of a new candidate point with respect to the markerdevice, the new candidate point being a new candidate for a position ofthe additional auxiliary point; b) checking whether the positions of thenew candidate point comply with the at least one constraint described bythe relative auxiliary point data; c) repeating steps a) and b) if theposition of the new candidate point does not comply with the at leastone constraint, or accepting the new candidate point as an additionalauxiliary point if the position of the candidate point does comply withthe at least one constraint.
 12. A program which, when it is running ona computer or is loaded onto a computer, causes the computer to performthe method according to claim 1; and/or a program storage medium onwhich the program is stored; and/or a computer on which the program isrunning or in the memory of which the program is loaded; and/or a signalwave, in particular a digital signal wave, carrying information whichrepresents the program.
 13. A navigation system for computer-assistedsurgery, comprising: the computer of claim 12; a detection device fordetecting the position of the main and auxiliary points and forgenerating detection signals which represent the position of the mainand auxiliary points and for supplying the detection signals to thecomputer of the preceding claim, the computer being designed todetermine the absolute point data and the relative point data on thebasis of the received detection signals in order to process the absolutedata in accordance with claim 1; and a user interface for receiving datafrom the computer in order to provide information to the user, thecomputer being designed to calculate the data in accordance with thedata processing method according to claim
 1. 14. A determination methodfor determining the position of a main plane of an anatomical body part,comprising the steps of: generating detection signals by detecting apointer, said pointer contacting a body part or a tool, called mainpoint detection tool, attached to the body part; determining, on thebasis of the detection signals, the absolute main point data andabsolute auxiliary point data as mentioned in claim 1, and thenperforming claim 1 on the basis of the determined absolute main pointdata and absolute auxiliary point data.
 15. The navigation system ofclaim 13 further comprising a tool, called main point detection tool,for detecting at least one actual main point, the plane point detectiontool comprising: a proximal side constituted to be fitted to a patientby contacting the patient along a first direction of length-wiseextension of the tool, the proximal side is constituted to be able to bealigned with a plane of an anatomical body part along the firstdirection of length-wise extension, the direction in which the proximalside faces and along which the tool is flexibly bendable for bringingthe tool into contact with the patient being a third direction ofextension of the tool; a distal side which comprises at least onepointer insertion member comprising a tapered recess for inserting thetip of a pointer, the at least one pointer insertion member beingprominently positioned in a first section of the distal side which is,in the direction of length-wise extension, closer to a first end of theplane point detection tool than to a second end; wherein the distal sidefurther comprises a second section having a flat surface and beingcloser to the second end than to the first end; both the proximal sideand distal side extending in the first direction and in a seconddirection of extension, the second direction being traverse to the firstdirection, the tool being flexibly bendable in the third direction whilebeing rigid in the second direction, the second direction being adirection of width-wise extension of the tool, the width-wise extensionof the tool being smaller than the length-wise extension.